比较 C 中链表中的值时的问题
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【中文标题】比较 C 中链表中的值时的问题【英文标题】:Issue when comparing a value in a linked list in C 【发布时间】:2015-01-19 02:50:04 【问题描述】:我正在使用 C 语言编写一个“简单”程序,在该程序中,我们创建了一个链表,其结构类似于电影,其中存储了标题、制作年份、评级 (1-5) 和指向下一个节点的指针。我们不允许在这个结构中添加任何东西,或者定义函数。
除此之外,我们(出于某种原因)需要在 main() 的主体中编写整个链表,这样就给问题增加了一层意大利面。无论如何,在这个程序中,我们应该让用户输入“U”来更新或输入“S”来搜索电影。更新符合您的预期,您输入标题、年份、评级。由此,我们应该将节点插入到链表的end。
我们的搜索应该遍历这个链接列表,如果找到匹配项,应该打印出电影、标题和年份。
虽然我的代码的更新部分有效,但我似乎无法让搜索工作。我正在使用一个名为 temp 的电影结构,它从 head 开始并遍历列表以尝试找到电影。通过 printf 运行一些测试后,我发现 temp 无论如何都只是一个空节点,无论我输入什么电影。
我假设这与我调用 malloc 的方式有关?还是与未正确分配节点有关?老实说,我不确定,不幸的是,实验室的 TA 也不知道出了什么问题 D:
这是我的代码:
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
struct movie_node
char title[250];
int year;
unsigned char rating;
struct movie_node *next;
;
typedef struct movie_node movie;
int main()
// variables
int loop = 1;
movie *head = NULL; // represents first
movie *current; // represents the last node
movie *temp; // used for traversing the linked list
head = malloc(sizeof(movie));
int amountOfMovies = 0; // increment after each addition
char choice; // either 'u' (update) or 's' (search)
// ask user for input
while(loop)
printf("%d Movie(s) in the database. Update or search (U/S): ", amountOfMovies);
scanf(" %c", &choice);
/* CHOICE 1, UPDATE */
if(choice == 'U')
// case 1, head is null, we must create a new node
if(head == NULL)
// get input
printf("Name of the movie: ");
scanf(" %[^\n]%*c", head->title);
printf("Year: ");
scanf("%d", &head->year);
printf("Rating: ");
scanf("%hhu", &head->rating);
head->next = NULL;
head = current; // set head to point to current
else
current = head;
// need to find where current is
while(current != NULL)
current = current->next;
// end while
// current is now at the null position, indicating empty node
current = malloc(sizeof(movie)); // allocate mem
// get user input
printf("Name of the movie: ");
scanf(" %[^\n]%*c", current->title);
printf("Year: ");
scanf("%d", ¤t->year);
printf("Rating: ");
scanf("%hhu", ¤t->rating);
current->next = NULL;
// end else
// output movie
printf("Movie \"%s\" is added to the database.\n", current->title);
amountOfMovies++; // increment amount of movies in database
else if(choice == 'S')
/* CHOICE 2, SEARCH */
// temp string
char tempTitle[250];
// flag to let us know if we found something
bool found = false;
// temp linked list to traverse
temp = head;
temp = malloc(sizeof(movie));
// ask user for input
printf("Name of movie: ");
scanf(" %[^\n]%*c", tempTitle);
printf("NAME OF MOVIE IN HEAD: %s\n", temp->title); // test, take out later
while(temp != NULL)
printf("NAME OF CURRENT MOVIE TO COMPARE TO: %s\n", temp->title); // test
if(strcmp(temp->title, tempTitle) == 0)
// match
printf("Year: %d\n", temp->year);
printf("Rating: %hhu\n", temp->rating);
found = true;
break;
else // no match so far
temp = temp->next;
printf("HAVEN'T FOUND MATCH, NEXT TITLE TO CHECK IS: %s\n", temp->title); // test print
found = false;
// end else
// end while
if(found == false) // no match confirmed
printf("Movie \"%s\" does not exist in the database.\n", tempTitle);
else // choice is invalid
loop = 0; // exit
// end if-else
// end while
// free all the nodes
return 0;
注意:我唯一还没有实现的是释放内存.. 我不是百分百确定我应该如何完成它。
非常感谢任何帮助..
【问题讨论】:
尝试为每个任务编写函数。这样您的代码将易于阅读和调试。 您的代码包括:if(head == NULL) printf("Name of the movie: "); scanf(" %[^\n]%*c", head->title);
这意味着您正在访问一个空指针,这是一个bad idea™。您应该只需要大量代码来进行输入。您可能需要创建一个函数来执行输入,并将指针传递给应读取数据的变量(结构)。您还应该针对在您的程序中输入“EOF”(control-D 或 control-Z)的用户强化您的代码(测试输入操作的结果)。
程序执行temp = head;
然后temp = malloc(sizeof(movie));
。这个逻辑是不是毫无意义?
【参考方案1】:
问题在于您的malloc()
电话。首先你要做:
movie *head = NULL;
// ...
head = malloc(sizeof(movie));
这意味着 head 不再为空,您将无法以您想要的方式插入第一部电影 - 将 malloc()
移动到其他位置。
其次,下面几行代码,你可以这样做:
current = head; // <- this is OK
// ...
current = malloc(sizeof(movie)); // allocate mem <- this is NOT OK, for the same reason as before
此外,您还可以阅读这样的电影标题:scanf("%249s", head->title)
。
如果你知道如何从那里出发,请告诉我。
除了代码中的问题之外,还有另一个问题:实验室的 TA 也不知道出了什么问题。
【讨论】:
【参考方案2】:样本修复
movie *new_node(void)//aggregate the creation of a new node
movie *node = malloc(sizeof(*node));//error check omit
printf("Name of the movie: ");
scanf(" %249[^\n]%*c", node->title);
printf("Year: ");
scanf("%d", &node->year);
printf("Rating: ");
scanf("%hhu", &node->rating);
node->next = NULL;
return node;
int main()
int loop = 1;
movie *head = NULL;
movie *current;
movie *temp;
//head = malloc(sizeof(movie));//"if(head == NULL) " don't work
int amountOfMovies = 0;
char choice;
while(loop)
printf("%d Movie(s) in the database. Update or search (U/S): ", amountOfMovies);
scanf(" %c", &choice);
if(choice == 'U')
if(head == NULL)
current = head = new_node();//need set to current
else
current = head;
while(current->next != NULL) //"current != NULL" can't link
current = current->next;
current = current->next = new_node();
printf("Movie \"%s\" is added to the database.\n", current->title);
amountOfMovies++;
else if(choice == 'S')
char tempTitle[250];
bool found = false;//need <stdbool.h>
temp = head;
//temp = malloc(sizeof(movie));//Unnecessary
printf("Name of movie: ");
scanf(" %249[^\n]%*c", tempTitle);
//printf("NAME OF MOVIE IN HEAD: %s\n", temp->title);
while(temp != NULL)
//printf("NAME OF CURRENT MOVIE TO COMPARE TO: %s\n", temp->title);
if(strcmp(temp->title, tempTitle) == 0)
printf("Year: %d\n", temp->year);
printf("Rating: %hhu\n", temp->rating);
found = true;
break;
else
temp = temp->next;
printf("HAVEN'T FOUND MATCH, NEXT TITLE TO CHECK IS: %s\n", temp->title);
//found = false;//Unnecessary
if(found == false)
printf("Movie \"%s\" does not exist in the database.\n", tempTitle);
else
loop = 0;
// free all the nodes
return 0;
【讨论】:
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